This invention relates generally to a method of fabricating clad metal ingots, more particularly clad slabs, in which a hollow body corresponding to the dimensions of the finished slab is in the form of a parallelepiped having at least one metal side plate of cladding material and thinner, metal side sheets. The body is inserted into an ingot mold or surface chill, a layer of fine-grained refractory insulating material fills the space between the bottom of the mold and the bottom of the hollow body as well as between the sides of the mold and the sides of the body, and the body is then charged with a molten base metal. The invention further relates to a hollow body for carrying out the method.
A method is disclosed in German Pat. No. 736 672 for weld or melt cladding, but the desired holohedral welding is not achieved during casting. If, during the charging of this known hollow body, the molten base metal is not extremely superheated, the very thick side plates of cladding material thereof function as a mold wall. During cooling and shrinking, the cast slab is permitted to set as a block in the mold at such a distance from the side plates as to form gaps. These gaps increase in size because the side plates, in turn, are heated unilaterally by the molten steel, thereby causing them to bulge outwardly and to warp like a pillow.
During cooling of the hollow body charged with the base metal in the surface chill and during reheating in the preheating furnace, the shrinking and expansion tendencies of the cooling base metal (standard steel) and of the cladding material (generally special steel) cause the gaps to increase in size. The gaps open at their terminal ends are accessible to the atmosphere which can penetrate between the cladding material and the base metal. This causes the interfaces thereof to oxidize, which oxidation is promoted by the elevated temperature. Thus, the bond between the cladding material and the base metal effected during the subsequent rolling operation weakens because of the formation of oxidation layers.
Normally, because of the very high thermal capacity of the side plates in comparison to the base metal, intimate bonding of cladding material and base metal resulting from integral casting can only be achieved with great difficulty and is only made possible by superheating the base metal. Due to the difference in heat dissipation there is less detachment between the cladding material and the base metal near the edges thereof as compared to the central portion. The base metal nevertheless contaminates as the result of the detachment of the cladding. Contamination of the base metal with Cr, Ni, C or Mn are completely unacceptable, even if there is only little detachment. Moreover, the cladding in this case is thinner and uneven.
To insure an adequately thick layer of coating material after the finished rolling of the plated slab in a subsequent operation, the thickness of the side plate cladding material usually amounts to 3 to 15% of the total thickness of the plated slab. In the method disclosed in German Pat. No. 736 672, this thickness ratio is even more likely greater than the upper limit indicated. However, with such a mass ratio, adequate heating and fusing of the initially normally tempered side plates by means of the heat content of the molten base metal, can only be achieved with difficulty.
When casting molten steel between two cladding sheets, it is assured than an intimate bond is made immediately following the casting by fusing the cladding material of the side plates. If superheated steel is employed, i.e., steel being cast at high temperatures, it has proven to be difficult in practice to adjust the temperature and the thicknesses of the material of the hollow body to one another in such a manner as to avoid dissolution of detachment during quenching. The side sheets, that is, the side faces of the hollow body not including the cladding material, would detach in accordance with German Pat. No. 736 672, because of their thinness. Moreover, superheating of the base metal requires a great deal of energy.
Furthermore, a method of producing clad ingots is disclosed in German Pat. No. 2 333 359, commonly owned herewith, in which cladding sheets are placed in a mold close to the walls thereof with the narrow side faces remaining freely accessible. During subsequent casting, these narrow side faces are intimately bonded with the base metal through heat shrinking and without dissolution, so that they provide a surface-side seal when the gap between the cladding sheet and the base metal develops.
However, this prior method is only suitable for relatively thick cladding sheets because of the amount of the desired sealing, i.e., the sealing through heat shrinking between the narrow side faces of the cladding sheets and the base metal is dependent upon the surface area of these narrow side faces and is improved as this area increases in size. However, since the coating material is clearly more costly than the base metal thereby giving rise to a minimization of the constituent amount of cladding material, this prior method has rarely been employed in the case of thin cladding plates. Moreover, it utilizes a great deal of energy.